There are numerous rain sensors for motor vehicles, for instance, as recited in German Published Patent Application No. 198 46 968. Essentially, the rain sensors consist of a housing in which a printed-circuit board having a transmitter and a receiver are located as optical components, and which is closed off by a light-conducting member mounted on the windshield of a motor vehicle. When in operation, the transmitter emits infrared radiation, which is coupled into the windshield by the light-conducting member mounted on the windshield of the motor vehicle. If moisture is present on the outside of the windshield, the radiation of the transmitter at this boundary surface is scattered toward the outside. If there is no moisture on this boundary surface, the radiation of the transmitter is completely reflected at this glass-air boundary surface. This completely reflected radiation is concentrated by the light-conducting member and focused on the receiver, which then emits a signal that may be taken as the degree of the moisture present on the windshield.
However, in the manufacture of motor vehicles, the light-conducting member is often already mounted on the windshield when the window is produced. In the final production phase, the housing with the printed-circuit board is then attached to the light-conducting member, by a clip-on connection, for instance, so that the light-conducting member closes the housing like a lid.